Wait... algae? Little green plants? Algae is a broad term encompassing all sorts of small plant-like organisms and causes a lot of confusion in casual conversation about coral reefs. It is used to describe the microscopic little dinoflagellates (zooxanthallae) that live in coral tissue and provide corals with food energy and colour. It is also used to describe the fleshy turf or plants that can come to dominate a reef after disturbance like removing the grazing fish, sedimentation, etc. Crustose coralline algae (CCA), the pink-ish layer encrusting on the dead coral skeleton in the centre of the above photo, is a red calcifying algae common on tropical reefs and more poleward coastal ecosystems.

One of CCA's many important ecological roles is being a kind of reef cement, or glue, that the binds fragments of reef together, as in the photo above from Butaritari, Kiribati. The experimental study found that elevated pCO2 concentrations inhibited the growth and recruitment of CCA. This preliminary finding - based on mesocosms conducted in Hawaii with a natural flow-through seawater and CO2-treated seawater - is a reminder that increasing CO2 concentrations could affect a broad array of calcifying organisms that play important ecological roles.